Securities - Schedule of Unrealized Losses on Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 Security	Dec. 31, 2014 Security
Obligations of Government Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 17,953	$ 19,362
Less than 12 months, Unrealized Losses	(33)	(77)
Less than 12 months, Number of Securities	3	5
12 months or more, Fair Value		19,757
12 months or more, Unrealized Losses		(238)
12 months or more, Number of Securities	0	4
Fair Value, Total	17,953	39,119
Unrealized Losses, Total	(33)	(315)
Number of Securities Total	3	9
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	260,103	78,786
Less than 12 months, Unrealized Losses	(681)	(386)
Less than 12 months, Number of Securities	55	19
12 months or more, Fair Value	168,693	240,055
12 months or more, Unrealized Losses	(3,523)	(6,618)
12 months or more, Number of Securities	32	43
Fair Value, Total	428,796	318,841
Unrealized Losses, Total	(4,204)	(7,004)
Number of Securities Total	87	62
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	68,439	12,615
Less than 12 months, Unrealized Losses	(450)	(96)
Less than 12 months, Number of Securities	79	15
12 months or more, Fair Value	20,777	61,548
12 months or more, Unrealized Losses	(445)	(952)
12 months or more, Number of Securities	31	93
Fair Value, Total	89,216	74,163
Unrealized Losses, Total	(895)	(1,048)
Number of Securities Total	110	108
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	53,122	2,969
Less than 12 months, Unrealized Losses	(113)	(31)
Less than 12 months, Number of Securities	16	1
12 months or more, Fair Value	1,978	4,573
12 months or more, Unrealized Losses	(8)	(137)
12 months or more, Number of Securities	1	2
Fair Value, Total	55,100	7,542
Unrealized Losses, Total	(121)	(168)
Number of Securities Total	17	3
Total Temporarily Impaired Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	399,617	113,732
Less than 12 months, Unrealized Losses	(1,277)	(590)
Less than 12 months, Number of Securities	153	40
12 months or more, Fair Value	191,448	325,933
12 months or more, Unrealized Losses	(3,976)	(7,945)
12 months or more, Number of Securities	64	142
Fair Value, Total	591,065	439,665
Unrealized Losses, Total	$ (5,253)	$ (8,535)
Number of Securities Total	217	182